1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) device, and more particularly to a structure of a memory cell in an MRAM device using magnetic memory cells which store data by the tunneling magnetoresistive effect.
2. Description of the Related Art
In recent years, there have been proposed many memory devices which store information based on a new principle. As one of such memory device, an MRAM device having both the non-volatility and the rapidity in which a plurality of memory cells including magnetic tunnel junction elements (which will be referred to as MTJ elements hereinafter) having a tunneling magnetoresistive effect are arranged in a matrix form is disclosed in, e.g., Roy Scheuerlein et. al. “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest pp. 128 to pp. 129.
The MTJ element has two magnetic layers which are generally referred as a recording layer and a fixed layer. When programming data in the MTJ element, a current is caused to flow through a write wiring, and a magnetic field in a predetermined direction is applied to the MTJ element, thereby changing the direction of magnetization of the recording layer.
Meanwhile, the most serious problem in the MRAM device is a reduction in a write current. The present inventors found that assuring a thermal stability of recorded information is an important problem as a result of an experiment of holding the reliability of the MTJ element. This prehistory will now be described hereinafter.
Under the present situation, a write current value of the MTJ element is as large as 8 to 10 mA. For a practical application, the write current value must be lowered to an allowable level. In the case of a test chip of the MRAM device on a 1K bit level manufactured by the present inventors by way of trial, the current write value is 8 to 10 mA as was expected.
Further, bit information retention characteristics of the MTJ element were examined. As a result, irrespective of a fact that criteria Ku×V/kB×T of the thermal stability of the recorded information which are usually considered in a magnetic medium of a hard disk storage apparatus are set to be not less than 80, some bit information were reversed. Here, V is a cubic volume of a recording layer of the MTJ element, kB is the Boltzmann constant, and T is an absolute temperature. In case of the MRAM device, Ku is given mainly based on a shape magnetic anisotropy as a general rule, and it is actually a sum of an anisotropic energy and an induced magnetic anisotropy.
For improving the thermal stability of the recorded bit information in order to prevent the bit information from being switched, Ku×U is usually set large. By doing so, however, the write current is increased.
In the MRAM device, it is desirable to achieve both a reduction in the write current and an assurance of the thermal stability of the recorded information as described above. In the prior art, however, a concrete design plan for this purpose is not proposed. The prehistory that this problem was found will now be described in detail hereinafter.
At present, a reported write current value of the MTJ element is at least approximately 8 mA if a cell width is approximately 0.6 μm and a cell length is approximately 1.2 μm.
Usually, a shape of a flat surface of the MTJ element is determined as a rectangular or an ellipse, the shape magnetic anisotropy is given to the MTJ element, a direction of magnetization of the MTJ element is stipulated, and the thermal stability of the recorded information is also given.
Ku×V is a product of a sum of the shape magnetic anisotropy and the induced magnetic anisotropy of the MTJ element, and a volume of the recording layer of the MTJ element. Here, the induced magnetic anisotropy of the recording layer is given in the same direction as that of the anisotropy based on a shape so as not to generate the dispersion of the anisotropy or the like. However, usually, NiFe used as a material of the recording layer has the induced magnetic anisotropy (several Oe) smaller than the anisotropic magnetic field based on a shape by a single digit, and it is considered that the thermal stability of the recorded information and the switching magnetic field are also substantially determined by the shape magnetic anisotropy.
The switching magnetic field Hsw required to rewrite magnetization information of the recording layer is substantially given by the following expression (1).Hsw=4π×Ms×t/F(Oe)  (1)
Here, Ms is a saturation magnetization of the recording layer, t is a thickness of the recording layer, and F is a width of the recording layer. Further, a sum Ku of the anisotropic energy based on a shape and the induced magnetic anisotropy is substantially given by the following expression (2).Ku=Hsw×Ms/2  (2)
As a method for reducing the write current, coating a conventional write wiring made of, e.g., Cu with a soft magnetic material such as NiFe and using it as a write wiring with a yoke is proposed in, e.g., Saied Tehrani, “Magneto resistive RAM”, 2001 IEDM short course. According to this method, the approximately twofold high-efficiency effect, i.e., the write current value can be reduced to approximately ½.
FIG. 1 shows an example of a structure of the write wiring with a yoke described in the above cited reference (“Magneto resistive RAM”), and FIG. 2 shows a result of examining write characteristics obtained by using the write wiring illustrated in FIG. 1. As shown in FIG. 1, the write wiring with a yoke 10 has a structure that a part of the periphery of a write wiring 11 made of Cu is coated with a yoke 12 made of a soft magnetic material such as NiFe.
In FIG. 2, characteristics A indicated by a solid line show a state that a width F of a recording layer is reduced and a switching magnetic field Hsw is increased as minuteness of an MTJ element is realized when a CoFeNi thin film having a film thickness of 2 nm is used as the recording layer. It is to be noted that characteristics B show a generated magnetic field when a conventional write wiring without a yoke is used, and characteristics C show a generated magnetic field when a write wiring with a yoke is used.
In case of using the conventional write wiring (characteristics B), since the generated magnetic field is larger than the switching magnetic field until 1/F is approximately 7, writing is possible. On the other hand, in case of using the conventional write wiring with a yoke (characteristics C), since the generated magnetic field is larger than the switching magnetic field even if 1/F exceeds approximately 7, writing is possible, but the generated magnetic field is smaller than the switching magnetic field when 1/F exceeds approximately 10.
As a result of examining the case that write wiring with a yoke formed by a prior art is used based on an experiment and a computer simulation, the approximately twofold high-efficiency effect was confirmed, and the write current can be reduced to 5 mA. However, this is the limit, and it is far from 1 to 2 mA which is a target value required for a practical application.
Furthermore, as a result of performing writing at a high speed by using a write current with a short pulse width of approximately 50 nsec, irregularities are generated in the required write current value, there is acquired only the reproducibility which is far below the reproducibility 90% obtained when writing is performed with a fixed write current.
On the other hand, even if Ku×V/kB×T of the recording layer is set to be not less than 80, some bit information is reversed. Although a cause is uncertain, the thermal stability of the recorded information is not determined by Ku×V, particles constituting the recording layer in a given defective cell undergo the thermal disturbance, i.e., Kcrysta×Vgrain (Kcrysta is a crystal magnetic anisotropy of a recording layer material, and Vgrain is a cubic volume of particles constituting the recording layer), and this becomes a factor of switching of the magnetization information of the cell. That is, it can be considered that there is a cell that Kcry×v (Kcry is a crystal magnetic anisotropic energy, and v is a cubic volume of one particle) determines the thermal stability of the recorded information.
FIG. 3 shows an example of an arrangement relationship between the conventional write wiring with a yoke and the MTJ element.
The write wiring with a yoke 10 has a structure that three surfaces of the write wiring 11, i.e., a bottom surface and both side surfaces are covered with a yoke 12 made of a magnetic material such as NiFe. Such a write wiring with a yoke 10 can generate a large magnetic field by using the same write current as compared with the regular write wiring without a yoke.
An MTJ element 20 has a structure that a non-magnetic layer 23 is sandwiched between a recording layer 21 and a fixed layer 22 each made of a magnetic layer. The fixed layer 22 is connected to a bit line (BL) 24.
FIG. 4 shows an example of a relationship between a distance from a yoke edge and a generated magnetic field when the write current is caused to flow through the write wiring 10 with the yoke depicted in FIG. 3. According to this magnetic field distribution, a certain degree of a large magnetic field is generated in the vicinity of the yoke edge. However, as distanced from the yoke edge, the magnetic field suddenly becomes small.
In order to solve such a problem, although using a material having a high crystal magnetic anisotropy in the recording layer was tried, it was revealed that the good outcome cannot be obtained by only using this material as a result of the experiment.
First, as the recording layer 21 in which information is written by the write wiring with the yoke 10 depicted in FIG. 3, a use of a soft magnetic material such as NiFe having the crystal magnetic anisotropy of approximately 103 erg/cc can be considered. In this case, a magnetic flux in the vicinity of the yoke edge is led to a central portion of the recording layer 21 by NiFe, and a large magnetic flux is led even to the center of the recording layer 21. In an experimental result, a magnetization direction in the recording layer 21 was able to be switched with a small write current value of approximately 2 mA. This can be considered because NiFe has a high magnetic permeability. It is to be noted that the shape magnetic anisotropy is approximately 104 erg/cc in this case.
However, as a result of examining data retention characteristics of the recording layer 21, a cell whose magnetization direction is switched was found, and it was revealed that there is a cell that the data retention characteristics of approximately 10 years cannot be expected. That can be considered because the magnetization direction of the recording layer 21 is switched due to reversal of the magnetization direction of magnetic material particles (NiFe particles) 31 at a less yoked cell central portion (central portion of the recording layer 21) from the shape anisotropy by the thermal disturbance, as shown in FIG. 5.
Thus, as the recording layer 21 in which information is written by the write wiring with the yoke 10 depicted in FIG. 3, a use of, e.g., a Co-based magnetic material having a larger crystal magnetic anisotropy can be considered. As a result of performing an experiment about a use of, e.g., CoNiFe having both the shape magnetic anisotropy and the crystal magnetic anisotropy being approximately 104 erg/cc as a material of the recording layer 21, the write current value used to switch the magnetization direction became as large as 15 mA, irregularities occurred in the write current for each cell, and the largest write current value exceeded 40 mA.
When CoNiFe was used, it was predicted that the write current value becomes approximately twofold of that when a NiFe-based material was used. However, the 7.5-fold write current is actually required as an average value, and a result was far from a reduction in the write current.
Moreover, since the magnetic flux in the vicinity of the yoke edge cannot be led to the center of the recording layer and the magnetization direction of the recording layer cannot be switched even if the magnetic field of the yoke edge is the same, a very large write current is required. That can be considered because the magnetic permeability of the Co-based material is not high. Additionally, as shown in FIG. 6, since irregularities are generated in directions of a crystal 32 of CoNiFe, i.e., since the crystal magnetic anisotropy due to a difference in crystal orientation in a film plane generates irregularities in intensity of the switching magnetic field for each cell in case of the Co-based material, it is estimated that great irregularities occur in the write current value for each cell.
Specifically, although the magnetic material is crystal-orientated in the vertical direction relative to the film plane, it cannot help becoming irregular in the film plane, and the crystal magnetic anisotropy competes with the shape magnetic anisotropy or the induced magnetic anisotropy given in a fixed direction of the recording layer. It is to be noted that an arrow in FIG. 6 indicates a direction of the shape magnetic anisotropy.
There are a cell that a direction of the shape magnetic anisotropy or the induced magnetic anisotropy and a direction of a crystal magnetic anisotropy are relatively aligned and a cell that such directions are not aligned, which becomes a factor of irregularities in the switching magnetic field for each cell. In particular, when a size of a cell is approximately 0.1 μm, the number of particles of the recording layer constituting one cell is approximately 10, which results in a further serious problem. It is to be noted that such a phenomenon is not observed in a NiFe-based material with the small crystal magnetic anisotropy, and hence no problem occurs.
To sum up, in order to assure the thermal stability of the recorded information, if the idea of using a material with the high crystal magnetic anisotropy to the recording layer is actually applied to the MRAM device, the write current value becomes large beyond expectation, and large irregularities are generated in the switching magnetic field. Therefore, there is demanded an MRAM device which can achieve both a reduction in the write current value and an assurance of the thermal stability of bit information, and has the high thermal stability and the high write efficiency.